Farmers believe as many as 25,000 sheep, cattle and pigs passed through three markets at the center of the outbreak during the week before a Friday ban on moving livestock within Britain.

"These figures show the sheer volume of movement," said Peter Kingwill, chairman of the Livestock Auctioneers' Association. "In terms of an outbreak, they are worrying."

Foot-and-mouth disease almost never infects humans, but it is highly contagious among cloven-footed animals like sheep, cows and pigs. It is not usually fatal in itself, but causes blisters on the mouth and feet, fever and loss of appetite.

Vaccines exist, but are quickly rendered ineffective by the development of new strains of the virus, so wholesale slaughter is used to contain the disease.

Five new cases of the highly contagious virus appeared Monday, nearly doubling the total number of infected areas to 12. The sudden escalation after a weekend with only one new case gripped the country with fresh fears that it could experience a repeat of a 1967 foot-and-mouth epidemic, when nearly half a million livestock were destroyed.

Officials elsewhere in Europe were growing increasingly worried that the disease could spread. The European Union was expected to extend a one-week ban on British exports of livestock, meat and dairy products due to expire Thursday.

"We are especially afraid the virus can cross the Channel by wind, or by birds, seagulls," said Veronique Bellemain, chief veterinary expert at the French agriculture ministry. "We can put controls on the Channel tunnel, but we can't control the winds."

As the impact of the disease spread, there were even calls from politicians to postpone a British national election, which many had expected to be called in May or earlier.

Two British broadsheet newspapers published maps Tuesday showing the spiderweb-like spread of the disease through England. Bonfires cast eerie glows in affected areas overnight as agriculture officials continued to oversee the burning of about 7,000 slaughtered animals.

Another 3,500 livestock have been killed in continental Europe, where no cases have been found but authorities fear the disease could spread.

Prime Minister Tony Blair on Monday called the outbreak "a dreadful blow" to a farming industry already struggling with mad cow disease, a fatal brain-wasting illness first identified in Britain.

Even if the foot-and-mouth outbreak is quickly contained, Chief Veterinary Officer Jim Scudamore has warned that an export ban  imposed two days after the first case was discovered at a slaughterhouse on Feb. 19  could remain in place for up to six months after eradication.

Efforts to trace the disease's progress have led investigators to three livestock markets  one in central England and two further north near Scotland  believed to have handled diseased animals.

Humans can carry the virus on their boots and clothing. The virus can also be airborne, transmitted from one animal to another, or contracted through contaminated feed.

Schools and a military training exercise have been disrupted in northern England because of fears of infection. Horse racing tracks have suspended races because horses  while immune to the disease  can carry the virus.

Hiking groups have scrapped country walks and fishing streams have been closed to anglers. Safari parks, zoos and nature reserves have closed or isolated animals susceptible to the disease.

Germany killed 350 imported sheep as a precaution, and the Netherlands slaughtered more than 3,200 animals brought from Britain before the outbreak was discovered. Spain ordered the destruction of 540 pigs imported from Britain, although the animals have shown no signs of foot-and-mouth disease.